A method is known from DE 44 27 688 A1 for checking the functional capacity of a tank ventilation system, wherein the tank ventilation system is sealed off in an airtight manner from the atmosphere by way of a check valve and then a tank ventilation valve is opened to establish a connection to the intake pipe of an internal combustion engine, with the result that a negative pressure builds up in the tank ventilation system. The dynamic pattern of the pressure drop in the tank ventilation system is used to evaluate the functional capacity of the tank ventilation system and to determine any lack of tightness or leaks present. The same evaluation takes place after the tank ventilation valve has been closed based on the analysis of the pressure build up taking place.
An activated carbon filter in the tank ventilation system collects the fuel gas leaving a fuel tank, thereby operating as a retention vessel. Opening the tank ventilation valve establishes a connection by way of a regeneration line between the retention vessel and the intake pipe, by way of which the hydrocarbons present in the retention vessel are supplied to the intake air of the internal combustion engine. The resulting sudden enrichment with hydrocarbons of the fuel/air mixture to be combusted results in a similarly sudden change in the air ratio lambda of the exhaust gas of the internal combustion engine. A generally present lambda regulating facility responds too slowly to such a sudden enrichment, which it is why it is proposed for example in DE 196 12 453 A1 that the enrichment of the fuel/air mixture occurring when the tank ventilation valve is opened should be taken into account when calculating the quantity of fuel to be introduced by way of the injection system into the internal combustion engine, in other words that the calculated injection time should be corrected by way of an additive value.
In order to be able to determine the additive correction value, it is necessary to determine the quantity of fuel supplied additionally by way of tank ventilation. Until now this has generally been done by way of the level of loading of the retention vessel with hydrocarbons. For the period of opening of the tank ventilation valve it is hereby assumed that the retention vessel discharges in a regular manner, in other words that a volume flow with a constant fuel concentration is supplied to the intake pipe. A constant additive correction value is determined accordingly from a loading value determined before discharge and is only changed after the retention vessel has been completely discharged and the loading has been determined once again. Particular structural embodiments of the retention vessel can however result in clear fluctuations in fuel concentration, which are not taken adequately into account by way of the constant additive correction value.